Process of preparing an open cell polyvinyl chloride sponge



PROCESS F PREPARING AN OPEN CELL POLYVINYL CHLQRIDE SPGNGE Mack FrancisFuller, Woodhury, Ni, assignor to E. I. du Pont de Nernours and Company,Wilmington, Del, acorporation of Delaware No Drawing. Application August22, 1956 p Serial No. 605,476

1 Claim. (Cl. 260-25) This invention relates to the manufacture ofexpanded cellular materials. More particularly, this invention relatesto the manufacture of expanded cellular polyvinyl chloride materials inbulk by means of a chemical blowing agent. 7

This application is a continuation-in-part of my prior applicationSerial No. 343,265, filed March 18, 1953.

The prior art discloses a wide variety of manufacturing techniques forthe production of expanded cellular polyvinyl chloride materials(sometimes called polyvinyl chloride sponge). The expansion of thepolyvinyl chloride may be accomplished mechanically by the use of aninert gas under pressure, or a chemical blowing agent may be used whichdecomposes upon heating to release a gas. Where the cellular polyvinylchloride material is intended for use in articles such as shoe innersoles, floats, or padding for athletic equipment, expansion of thepolyvinyl chloride is normally carried out at superatmospheric pressurein aclosed mold. Cellular materials produced in this fashion arecharacterized by individual cells and are referred to as unicellular orclosed cell sponge. Cellular materials useful for making toys, padding,cushioning, coat interliners, etc. are prepared under atmosphericpressure conditions; they may contain some closed cells but generallyhave a continuous, interconnected cell structure and are calledopen-cell sponge.

It is an object of the present invention to provide a novel process forthe preparation of open-cell polyvinyl chloride sponge without the useof gas-tight molds or pressure equipment. It is a further object of theinvention to provide a process of the aforementioned type which can becarried out at moderate temperatures and at atmospheric pressureconditions. It is a still further object of the invention to provide aprocess which is convenient, economical, and consistently produces afine, high-quality sponge product. These and other objects will becomeapparent from a consideration of the ensuing specification and claim.

' Broadly stated, my invention involves the use, in an atmosphericblowing process, of a chemical blowing agent which will undergo thermaldecomposition and release to a gel state. By plastisol, I mean thepolyvinyl chloride resin combined with sufiicient plasticizer to form afluid composition and containing any of the customary compoundingingredients such as stabilizers, pigments,

fillers, etc. This technique differs distinctly from prior artatmospheric blowing processes wherein the polyvinyl chloride plastisolis heated to a temperature at which'it gels'and fluxes and then heatedfurther to decompose the blowing agent, whereby the molten fluxed massis then expanded.

Formerly, it was believed that only a fluxed plastisol had suflicientviscosity to retain the gas formed by decomposition of the blowing agentat atmospheric pressure. I have now found that I can obtain high qualitypolyvinyl chloride sponge products with an atmospheric blow- Example 1 Aplastisol was prepared by stirring together the following ingredients:

Parts Plastisol grade polyvinyl chloride Di-Z-ethylhexyl phthalate(plasticizer) Basic lead carbonate 5 Dinitrosopentamethylenetetramine(blowing agent) 4 Citric acid 2 The plastisol was poured into a papertray to a depth of A and heated in an oven at 100 C. for 30 min. Thetray was then transferred to an oven at C. and heated for 20 min. Theexpanded composition thus obtained had a volume 2 /2 times that of theoriginal plastisol, had a medium-fine, uniform, continuous cellstructure, and was very light cream in color.

Example 2 A plastisol was prepared by stirring together the followingingredients:

Parts Plastisol grade polyvinyl chloride 100 Di-Z-ethylhexyl phthalate(plasticizer) 100 Chlorinated paraffin (40% chlorine) (plasticizer) 50Basic lead carbonate 5 N,N dimethyl, N,N' dinitroso terephthalamide(blowing agent) 10 The plastisol was poured into an open aluminum trayto a depth of /s" and heated in an oven at 100 C. for 30 minutes. Thetray was then transferred to an oven at 150 C. and heated for 30minutes. The expanded composition thus obtained was a white, tough,resilient sponge, which was one inch thick and free of objectionableodor, and had afine, uniform, continuous cell structure.

The invention has been exemplified for the preparation of cellularpolyvinyl chloride materials since I envision that it will have itsgreatest utility in. this connection. It will be readily apparent,however, that my novel process is useful for the expansion of other anddifferent thermoplastic organic resins, especially mixtures of polyvinylchloride with minor amounts of copolymers of vinyl chloride with vinylacetate, diethyl maleate, diethyl fumarate, vinylidene chloride and andthe like.

My invention is not limited to any. single blowing agent. As is apparentfrom the foregoing examples, numerous blowing agents are'operative. Theonly critical limitation on the suitability of the blowing agent is thatit be capable of decomposing and releasing, gas rapidlyto foam thepolyvinyl chloride plastisol at a temperature below that at whichthe-plastisol passes from a liquid to a gel state.

In order that decomposition of the blowing agent may be completed beforegelation of the plastisol occurs, I normally foam the plastisol in anoven with an air temperature set substantially below the known gelpoint. Under certain conditions, however, for example, foaming inthick-walled molds or in relatively thin layers on insulating substratasuch as paper or fabric, the heat transfer characteristics of the systempermit complete decomposition of the blowing agent before the plastisolis gelled, even though the air temperature of the oven may be higherthan the plastisol gelation temperature. A critical feature of thisinvention is that heating of the plastisol be conducted in such a mannerthat the blowing agent be essentially completely decomposed before thegelation temperature of the plastisol is reached. As a highlyplasticized polyvinyl chloride composition, the socalled plastisol, israised from ordinary room temperatures, it remains in a definite liquidstate (though usually increasing in viscosity) until a temperature isreached which causes the plastisol to pass from a mobile, flowable stateto a relatively immobile or gel state. The gel state is achievedprincipally as a result of temperature but also is affected by the timeat which a composition is heated at a given temperature. Thesusceptibility of a resin to solvation by a plasticizer detennines thetemperature at which a composition ceases to be a liquid and becomes agel. As the composition is heated further, it passes from the gel stageto the fluxed, or fused, stage in which the resin and plasticizer becomedissolved in each other in a single phase formation. In this last stagethe composition is again mobile to some extent at elevated temperaturesand to some extent foamable. However, the mobility and foamability ofthe composition in the fused stage is much less than the liquidplastisol prior to gelation. Prior art blowing techniques at atmosphericpressure are nevertheless all directed to expansion of the plastisol inthe fused stage. The present invention takes advantage of the greatermobility and foamability of the unfiuxed plastisol.

My novel concept contravenes what has heretofore been the acceptedtheory and practice in the art of preparing cellular polyvinyl chloridematerials at atmospheric pressure. It was formerly believed that foamingof the liquid plastisol at atmospheric pressure prior to gelation andfiuxing would yield very low quality sponge products, perhaps evencompletely unexpanded. The prior art suggests that the gaseousdecomposition products of the blowing agent at atmospheric pressurewould quickly dissipate if the plastisol were ungelled, unfluxed, andstill liquid when the blowing agent decomposes. Not only have I foundthis not to be the case, but I have succeeded in preparing spongeproducts of a relatively low density (i.e., highly expanded) in thicksections. With my novel process, I have prepared plastic sponge insections up to 8 inches thick having densities as low as 4 pounds percubic foot at atmospheric pressure and without the use of costly andcomplicated pressure equipment.

The basic components of the plastisol which I use in my process are thepolyvinyl chloride resin, a plasticizer, and a chemical blowing agentwhich decomposes at a temperature lower than the gelation temperature ofthe plastisol. Other compounding ingredients may be present in theplastisol, but are not critical. Thus, the plastisol may contain suchadditives as stabilizers, pigments, anti'oxidants, activators, etc.

The invention is not limited to any particular plasticizer or class ofplasticizers. Any conventional plasticizer for the polyvinyl chloride issuitable provided it meets the customary standards of compatibility,stability, etc. In the case of polyvinyl chloride resins, there are manywell-known plasticizers that have long been in use such asdidecylphthalate, dioctyl sebacate, di-Z-ethylhexyl phthalate, dioctyladipate, etc. Numerous polymeric plasticizers are operative, and are inmany cases preferred. The quantity of plasticizer used may vary betweenabout 50 and 160 parts per 100 parts of resin,

4 depending upon the properties desired in .the product; it is typicallyin the neighborhood of parts of plasticizer per 100 parts of thepolyvinyl chloride resin.

The precise temperature at which the blowing agent commences to undergothermal decomposition is not critical provided only that decompositionis substantially complete at a temperature below the gelationtemperature of the plastisol. The latter temperature will vary somewhatfrom case to case depending upon the particular polyvinyl chloride resinand the amount of plasticizer used. With most polyvinyl chlorideplastisols compounded for foaming, gelation usually occurs in the rangeof about 100l5 0 C. and most frequently at some temperature betweenabout -140 C. The preparation of expanded cellular polyvinyl chloridematerials according to the present invention thus requires a blowingagent which undergoes, in the plastisol composition, extensive thermaldecomposition at temperatures below about C. For storage and shipmentpurposes, the blowing agent will desirably be relatively stable atordinary room temperatures and will not commence to undergo anysubstantial thermal decomposition until the temperature of theenvironment exceeds such temperatures, i.e., at about 60 C. or higher.There are numerous blowing agents which fit this description including,for example, N,N-dinitroso N,N-dimethyl terephthalamide; N,N-dinitrosoN,N-dimethyl adipainide; N,N-dinitroso N,N-dimethyl succinamide;N,N-dinitroso, N,N'-dimethyl sebacamide; ethylene bis (-nitrosobenzamide); p-(t-butyl) benzazide', p-carbomethoxybenzazide;N,N'-dinitroso N,N'-dimethyl 4,4'-bibenzamide; a,a azobis(isobutyronitrile); acidactivated dinitrosopentamethylenetetramine, etc.

The proportion of the blowing agent in the plastisol may also varywidely depending upon the degree of expansion desired in the cellularproduct. In a typical case, the plastisol will contain about 2-14 partsof blowing agent, by weight, per 100 parts of the resin. Up to 21 partsof blowing agent per 100 parts of resin is generally operable.

The expanded cellular products produced by the process of the presentinvention are useful for a wide variety of cushioning and insulatingfunctions including, for example, upholstery filler, crash pads, paddingand insulation for clothing, acoustical insulation, rug underlay and thelike.

Having thus described my invention, I intend to be linited only by thefollowing claim.

I claim:

A process for preparing an open-cell polyvinyl chloride sponge whichincludes the steps of forming a fluid plastisol by mixing a polyvinylchloride resin and a suitable plasticizer therefor, adding a compound tosaid plastisol which wil decompose upon heating to rapidly release a gasat a temperature between about 60 and 130 C., heating the mixture of theplastisol and the compound at atmospheric pressure to a temperaturebetween about 60 and 130 C. to decompose the compound and release a gaswhich will transform the liquid plastisol into a greatly expandedfragile foam, and thereafter heating the fragile foam to a highertemperature between about 130 C. and the flux point of the plastisol togel the plastisol and flux the polyvinyl chloride with the plasticizer.

References Cited in the file of this patent UNITED STATES PATENTS2,447,056 Cooper Aug. 17, 1948 2,525,880 Feldman Oct. 17, 1950 2,525,966Smith et al Oct. 17, 1950

